Electronic ballast for a discharge lamp

ABSTRACT

An electronic ballast for a discharge lamp has a compact arrangement of electric parts. The ballast includes an AC-DC converter and an inverter giving a high frequency output voltage for operating the lamp. The inverter includes switching elements, a control circuit, and an output transformer for connection with the lamp. A single circuit board mounts the electric parts forming the converter and the inverter. The circuit board is accommodated within a tubular casing together with a dielectric sheet which is provided for insulating the circuit board as well as the electric parts from the casing. The output transformer is mounted on a top surface of the circuit board together with some of the other electric parts. The ballast includes chip components forming a detection circuit for lamp condition, and the circuit board mounts the chip components on a bottom surface at a location immediately opposite of the output transformer.

TECHNICAL FIELD

The present invention relates to an electronic ballast for a dischargelamp, and more particularly to an assembly of the electronic ballastwhich is added with a sophisticated function but is made compact enoughto be installed in a limited space.

BACKGROUND ART

A prior art electronic ballast for a discharge lamp has been designed toassemble various electric parts forming the circuits of the ballast intoa single casing. In the prior ballast, it has been a practice to use acircuit board which mounts on its one surface the electric parts and isassembled into the casing. When the ballast is required to have an addedfunction of, for example, controlling the operation of the dischargelamp based upon the operating condition of the discharge lamp, thecircuit board is designed to mount a corresponding number of partsforming a detection circuit of detecting the operating condition of thedischarge lamp, and therefore requires an extra space for accommodatingthe parts of the detection circuit. Further, when the ballast isdesigned to have an inverter with an output transformer which isinherently large, the circuit board has to be correspondingly largeenough to mount the output transformer together with the other electricparts. Accordingly, the prior art ballast has an underlying problem inthat it is difficult to be made compact for easy installation in alimited space when the ballast is equipped with the added function ofcontrolling the operation of the discharge lamp based upon the detectedoperating condition thereof and includes the output transformer.

DISCLOSURE OF THE INVENTION

The above problem has been solved by the present invention whichprovides an improved architecture of enabling compact arrangement of theelectric parts forming the circuits of the ballast when the ballast isequipped with a detection circuit for detection of the operatingcondition of the discharge lamp and includes an output transformer. Anelectronic ballast in accordance with the present invention comprises anAC-DC converter adapted to be connected to receive a source AC voltageand convert the source AC voltage into a DC voltage. An inverter isprovided to give a high frequency output voltage from the DC voltage foroperating the discharge lamp. The inverter includes at least oneswitching element, a control circuit for the switching element, and theoutput transformer adapted to be connected to the discharge lamp. Thecontrol circuit activates the switching element to repetitivelyinterrupt the DC voltage to provide the high frequency output voltage tothe discharge lamp through the output transformer. Included in theballast is a single circuit board mounting thereon electric parts whichconstitute the converter and the inverter. The circuit board mountingthe electric parts is accommodated within a tubular casing together witha dielectric sheet which is provided for insulating the circuit board aswell as the electric parts from the casing. The output transformer ismounted on a top surface of the circuit board together with some of theother electric parts. The characterizing feature of the presentinvention resides in that the ballast includes a plurality of chipcomponents forming the detection circuit for detection of an operatingcondition of the discharge lamp, and that the circuit board mounts thechip components of the detection circuit on a bottom surface at alocation immediately opposite of the output transformer.

Thus, the circuit board can be insulated successfully from the casing bythe use of the dielectric sheet, thereby minimizing a spacing betweenthe circuit board and the casing to achieve a low profile structure forthe assembly of the ballast. Further, the dead space left immediatelybehind the output transformer of inherently large configuration can bebest utilized to mount the chip components forming the detection circuitresponsible for achieving an added function of the ballast, therebyachieving compact arrangement of the electric parts on the circuit boardfor reducing a size requirement of the circuit board and thereforecontributing to a compact design of the ballast equipped with the outputtransformer and with the detection circuit.

Accordingly, it is a primary object of the present invention to providean electronic ballast for a discharge lamp which is capable of beingdesigned into a compact structure sufficient to be installed in alimited space even with the provision of the output transformer and theadded detection circuit for detection of the operating condition of thedischarge lamp.

In a preferred version, the detection circuit includes a resistornetwork composed of chip resistors for dividing the output voltage ofthe inverter into a detection voltage which is fed to the controlcircuit for control of the inverter. Since the output voltage is dividedinto the detection voltage being fed to the control circuit, the circuitboard can be designed to have less restriction with regard to dielectricdistance for a path of feeding the detection voltage to the controlcircuit on the bottom of the circuit board, thereby realizing a compactarrangement of the circuit.

The detection circuit may also include a chip capacitor which issurface-mounted on the bottom surface of the circuit board together withchip resistors forming the resistor network.

Also, the ballast may include another detection circuit in the form of aresistor network composed of chip resistors for detection of whether thedischarge lamp is connected to the inverter. The chip resistors are alsosurface-mounted on the bottom of the circuit board immediately oppositeof the output transformer.

The output transformer is preferably in the form of a flatconfiguration. In order to give sufficient electrical insulation to theoutput transformer, the dielectric sheet is configured to have anextension which covers the entire top face of the output transformer.With this insulation by the extension of the dielectric sheet, nofurther extra spacing is required between the output transformer and thecasing, thereby reducing an overall height of the assembly. Therefore,it is another object of the present invention to provide the electricballast which is capable of realizing a low profile structure of theassembly.

The inverter includes a smoothing capacitor which accumulates thereinthe DC voltage from the converter, and may further include a set ofcapacitors which generate a relatively large amount of heat. The circuitboard is preferred to be of an elongated configuration and designed tomount the output transformer and the set of the capacitors on its topsurface at its opposite longitudinal ends thereof, respectively. Thesmoothing capacitor is mounted on the top surface of the circuit boardintermediate between the longitudinal ends thereof so as to be lessaffected thermally from the output transformer as well as the set of thecapacitors. Thus, the smoothing capacitor of which operation life may beshortened by exposure to the heat can be well protected thermally fromthe heat generating parts, enjoying a prolonged operation life.

Preferably, the casing is of an elongated configuration and is composedof a lower case having a base and a pair of lower side walls upstandingfrom the opposite lateral sides of the base, and an upper case having atop and a pair of upper side walls depending from the opposite lateralsides of the top to overlap the lower side walls, respectively. Theswitching element is molded into a flat resin package and is mounted onthe circuit board intermediate the opposite longitudinal ends and at onelateral side of the circuit board so as to project on the top surface ofthe circuit board. The resin package has a major surface being held inintimate contact with one of the lower side walls for escaping heatgenerated at the switching element to the lower case. Thus, the heatradiation of the switching element can be made by best utilization ofthe lower case. Further, due to the heat radiation effect, the switchingelement can be located rather adjacent to the smoothing capacitorwithout thermally affecting the latter, which assures a compactarrangement of the parts on the circuit board.

The one of the lower side walls may be formed with a recess of whichbottom projects inwardly of the lower case. The recess is locatedintermediate the longitudinal ends of the lower case. The package of theswitching element is secured to the bottom of the recess by means of aspring clip for making the major surface of the package intimate contactwith the bottom of the recess. The spring clip is shaped into a U-shapedconfiguration having a pair of resilient legs connected by a thin flatmember for pinching an upper portion of the package and the bottom ofsaid recess between the legs. The thin flat member is kept in intimatedcontact with a top of the package as well as with the top of the uppercase for escaping the heat generated at the package also to the uppercase through the member. The resilient leg is dimensioned to have athickness smaller than a depth of the recess. Thus, the spring clip canfacilitate to secure the package in intimate contact with the bottom ofthe recess. Further, the member of the spring clip acts to escape theheat from the package also to the upper case, improving the heatradiation of the package, which is a further object of the presentinvention. In addition, the leg of the spring clip is received withinthe depth of the recess so that the spring clip does not add an extrawidth to the casing or the entire assembly of the ballast.

Preferably, the inverter includes a pair of switching elements connectedin series across an output of the AC-DC converter. The control circuitincludes a driver having a level-shift circuit for providing a highdriving voltage to turn on a high side one of the switching elements.The control circuit is integrated together with the driver into a singlechip which is mounted on the bottom surface of the circuit board at alocation intermediate the longitudinal ends of the circuit board. Withthis architecture of integrating the control circuit and the driver intothe single chip, a plurality of discrete components forming the controlcircuit and the driver can be assembled into the single part forreducing a number of the overall parts mounted on the circuit board andtherefore realizing a compact arrangement of the ballast.

The inverter includes a plurality of capacitors each of which may be inthe form of a film capacitor. Also, the circuit board is preferablyprovided on its top surface at the longitudinal ends respectively withan input terminal socket for connection of the converter to the sourceAC voltage and an output terminal socket for connection of the outputtransformer to the discharge lamp. The output transformer is disposedadjacent to the output terminal, while the film capacitors are mountedcollectively on the circuit board between the output transformer and theoutput terminal socket. Thus, the film capacitors which develop onlyrelatively low amount of heat can be closely packed into a small spacewithout giving substantial thermal effect to each other, therebyincreasing mounting density of the parts to make the assembly compact.

The circuit board may have an approximately uniform width along thelength thereof. The width is selected to be nearly equal to a width ofthe output transformer which is greatest among all of the electricparts. Thus, the assembly can be reduced to as less as the width of theoutput transformer for achieving a compact design for the assembly.

The circuit board is formed on its bottom surface with a ground line forthe inverter, and with a main current path carrying a high frequencycurrent to the output transformer. The main current path includes a pairof main current lines major portions of which run in close parallelrelation to each other. The chip of the control circuit is mounted onthe bottom surface of the circuit board to be isolated from the maincurrent lines by the ground line so that the control circuit can beprotected from being affected by noises occurring in the main currentpath. Further, since the major portions of the main current lines run inclose parallel relation to each other, it is readily possible tominimize an area confined by the main current path, thereby reducing anamount of radiation noise therefrom.

The circuit board is preferably separated from an array in which aplurality of the circuit boards are arranged side-by-side with aseparation line between the adjacent ones of the circuit boards. Eachcircuit board has a reduced-in-width section within the length thereofto define a slit along the separation line between the adjacent circuitboards. The inverter includes at least one capacitor in the form of achip-type ceramic capacitor. The chip-type ceramic capacitor is mountedon the reduced-in-width section. Since the reduced-in-width section canbe less affected by a stress developed when separating the circuit boardfrom the array, the chip-type ceramic capacitor can be well protectedfrom the stress so as to be kept intact for reliability of the inverter.

The electric parts includes a plurality of chips each having terminalson opposite ends thereof, respectively. The chips are mounted on thebottom surface of the circuit board with all of the terminals orientedin a width direction of said circuit board. Thus, the terminals of thechips can be equally and reliably soldered to the circuit board when thecircuit board is fed through a soldering bath along the length of thecircuit board.

The dielectric sheet is preferred to have a plurality of studs forsupporting the circuit board. With the provision of the studs, thecircuit board can be stably supported on the dielectric sheet in such amanner as to avoid unnecessary engagement between the leads of someelectric parts and the dielectric sheet which might otherwise damage thedielectric sheet.

The case may be provided with positioning means for holding the circuitboard in correct position for easy assembly of the ballast.

The output transformer is preferred to include a first auxiliary windingwhich provides a driving voltage for energizing the control circuit, asecond auxiliary winding which provides a detection output indicative ofthe high frequency output voltage for monitoring the operation of thedischarge lamp, and a third winding which provides a preheat current tofilaments of the discharge lamp. Thus, the output transformer alone canprovide versatile functions, which reduces the number of the discretecomponents for the inverter, leading to a compact assembly of theballast.

Preferably, the output transformer is a leakage transformer having aprimary winding connected in circuit to flow a high frequency current,and a secondary winding which is magnetically coupled to the primarywinding to induce the high frequency output voltage being applied to thedischarge lamp. With the use of the output transformer of the leakagetype, a leakage inductance can be best utilized as an oscillationinducing inductance for the inverter operation, requiring no additionalseparate inductance and therefore reducing the number of the overallcomponents of the inverter for compact arrangement of the ballast.Further, the first auxiliary winding is coupled to the first auxiliarywinding so as to provide the driving voltage for energizing the controlcircuit. Thus, the control circuit can be stably supplied with theenergy irrespective of the operating condition of the discharge lamp.Also, the second auxiliary winding is coupled to the secondary windingso as to provide the detection output indicative of the high frequencyoutput voltage such that the second auxiliary winding can provide thedetection output reliable enough for detection of the operatingcondition of the discharge lamp.

The AC-DC converter includes a rectifier providing the rectified DCvoltage. The control circuit is preferred to include a dropping resistorwhich is connected to the rectifier for deriving a DC voltage forenergizing the control circuit at the time of starting the inverter.Thus, the dropping resistor is integrated into the control circuit toreduce the number of the discrete components.

Most preferably, the inverter is of a charge-pump type having acapacitor which is connected in a path of receiving an input currentfrom said AC-DC converter and flowing the input current through anoscillating element of the inverter and through the switching elementfor restraining harmonics in the input current from the source ACvoltage. With the use of the charge-pump type inverter, it is possibleto restrain the harmonics of the input current from the source ACvoltage and therefore improve power factor of the ballast withoutrequiring any external harmonic filter, contributing to reduce thenumber of the circuit components for the inverter capable of suppressingthe harmonics.

The ballast may include an adjusting means mounted on the circuit boardto adjust output characteristics of the discharge lamp for operating thedischarge lamp successfully even when the ballast is energized by thesource AC voltage of different voltages, thereby enabling to operate thelamp consistently from the differing AC voltage sources with the use ofthe common ballast.

The adjustor means can operate to adjust a lamp start output power and alamp operating output power fed to the discharge lamp respectively atthe time of starting the lamp and during the steady-state operation ofthe lamp, thereby minimizing electrical stress applied to the electriccomponents of the inverter and therefore giving reliability thereto, inaddition to facilitating the circuit design of the inverter.

These and still other objects and advantageous features of the presentinvention will become more apparent from the following description ofthe preferred embodiment when taken in conjunction with the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of an electronic ballast for adischarge lamp in accordance with a preferred embodiment;

FIG. 2 is a perspective view showing the bottom of a circuit boardutilized in the above ballast;

FIG. 3 is a circuit diagram of the above ballast;

FIGS. 4A to 4F are diagrams showing the operation of an inverter in theabove ballast;

FIG. 5 is a block diagram of a control circuit utilized for control theoperation of the inverter;

FIG. 6 is a circuit diagram of a driver incorporated in the controlcircuit;

FIG. 7 is a time chart illustrating the operation of the driver;

FIG. 8 is a partial top view of the circuit board;

FIG. 9 is a partial perspective view illustrating switching transistorssecured to a case of the ballast;

FIG. 10 illustrates a spring clip for securing the switchingtransistors;

FIG. 11 is a sectional view illustrating the switching transistorssecured to the case;

FIG. 12 is a schematic plan view illustrating a portion of the bottom ofthe circuit board;

FIG. 13 is a schematic plan view illustrating a portion of the bottom ofthe circuit board;

FIG. 14 is a perspective view illustrating end portions of a lowercasing;

FIG. 16 is a schematic top view illustrating the circuit board supportedin position to the lower casing;

FIGS. 16 and 17 are schematic top views illustrating modifiedarrangements for supporting the circuit board to the lower casing,respectively;

FIG. 18 illustrates an end portion of the circuit board engaged with apositioning stand of the lower case according to the arrangement of FIG.17;

FIG. 19 is a plan view illustrating an array of the circuit boards whichare to be separated from each other;

FIG. 20 is a sectional view of the array; and

FIG. 21 a plan view of a portion of the circuit board in accordance witha further modification of the embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIG. 1, there is shown an electronic ballast inaccordance with a preferred embodiment of the present invention. Theballast comprises a circuit board 10 mounting various electric partsforming an electric circuit of the ballast, a tubular casing composed ofa lower case 210 and an upper case 220 to enclose the circuit board, anda dielectric sheet 230 covering the circuit board 10 as well as theelectric parts for insulating the same from the casing. The casing ismade of a metal such as steel and aluminum. The circuit board 10 is ofan elongated configuration and provided on its top surface at itsopposite longitudinal ends respectively with an input terminal socket 20for wiring connection with an AC mains 21 and with an output terminalsocket 30 for wiring connection with discharge lamps LA1 and LA2. Thecircuit board is made of a phenol resin impregnated paper or a compositeepoxy material (CEM).

FIG. 3 shows the ballast circuit which is designed to operate twodischarge lamps LA1 and LA2. The ballast circuit has an AC-DC converter40 in the form of a full-wave diode-bridge rectifier which is connectedthrough the input terminal socket 20 to receive a source AC voltage fromthe AC mains 21 and convert the source AC voltage into a pulsating DCvoltage. The converter 40 is connected to the AC mains 21 through afiltering capacitor 41, and a filter 42 composed of a common-mode choke43, and a normal-mode choke 44. The pulsating DC voltage from therectifier 40 is smoothed by a step-down chopper 50 composed of asmoothing capacitor 51, an inductor 52, diodes 53 to 55, and a switchingelement 72. The smoothing capacitor 51 is connected in series with theinductor 52 and diode 53 across the rectifier 40 in a parallel relationrespectively with capacitors 61 and 62. The circuit includes an inverter70 which converts the DC voltage into a high frequency output AC voltageto be applied to the discharge lamps LA1 and LA2. The inverter 70comprises a series connected pair of switching elements 71 and 72connected across the rectifier 40 in series with a series combination ofdiodes 73 and 74, and a leakage-type output transformer 80 having aprimary winding 81 and a secondary winding 82. The primary winding 81 isconnected across the switching element 71 in series with an inductance83 which is given by a leakage inductance of the transformer, a DCblocking capacitor 75 and diode 74. The secondary winding 82 isconnected across a series combination of the discharge lamps LA1 and LA2in parallel relation to an oscillation-inducing capacitor 77. Theswitching element 72 is connected in series with the primary winding 81,leakage inductance 83, capacitor 75, and diode 73 across the rectifier40 for drawing in an input current through the switching element 72. Adistortion improving capacitor 76 is connected across diode 74.

Switching elements 71 and 72, each realized by a MOSFET having aninherent parasitic diode permitting a reverse current flow, arecontrolled to turn on repetitively at a high frequency in an alternatefashion with each other so as to generate the high frequency voltage ata resonance circuit composed of the leakage inductance 83, dischargelamps LA1, LA2, and capacitor 77, thereby applying the high frequencyoutput voltage to operate the discharge lamps.

Switching elements 71 and 72 are controlled to turn on and off by acontrol circuit 100 including a driver IC 110 which provides drivingvoltages to gates of switching elements 71 and 72 respectively throughresistors 77 and 78. Capacitor 61 is connected in parallel with theseries combination of smoothing capacitor 51, inductor 52, and diode 53of the step-down chopper 50 to bypass the high frequency output of theinverter, while capacitor 62 is connected across the rectifier 40 tobypass the high frequency output of the inverter for reducing a stressat the rectifier 40.

The smoothing capacitor 51 is connected in series with a switchingelement 64 and a resistor 65 across the rectifier 40. The switchingelement 64 is controlled by the control circuit 100 to be kept turnedoff only at the time of energizing the inverter in order to restrict arushing current from flowing into the inverter for protection of thecircuit. After the smoothing capacitor 51 is charged, the switchingelement 64 is turned off to enable the inverter operation of providingthe high frequency output voltage.

The output transformer 80 additionally includes a first auxiliarywinding 84, a second auxiliary winding 85, and a third auxiliary winding86 which induce individual voltages proportion to the high frequencyoutput voltage of the inverter. The first auxiliary winding 84 iscoupled to the primary winding 81 to provide a power through a diode 66to the control circuit. Since the first auxiliary winding 84 is coupledon the side of the primary winding 81, it can supply the power to thecontrol circuit 100 stably without being affected by the condition ofthe discharge lamps coupled on the side of the secondary winding 82. Thesecond auxiliary winding 85 is coupled to the secondary winding 82 toprovide a voltage corresponding to the high frequency output voltagebeing applied to the discharge lamps and therefore give a reliablemonitoring of the output voltage. The third auxiliary winding 86 iscoupled to the secondary winding 82 and is cooperative with a capacitor87 to provide a preheating current to the filaments of the dischargelamps for facilitating the start-up of the discharge lamps.Incidentally, a capacitor 88 is connected to one end of the secondarywinding 82, and a capacitor 89 is connected across the one dischargelamp LA1.

As shown in FIGS. 4A to 4F, the inverter operates to repeat six modesmany times during one complete cycle of the AC voltage to suppressharmonics, i.e., input current distortion in the input current from theAC voltage source to the inverter. In these figures, output transformer80, discharge lamps LA1 and LA2 as well as the associated parts arecollectively referred to as a load circuit RL, and also inductor 52,diodes 53 to 55, switching element 64, and resistor 65 are omitted forsimplicity.

Mode 1 (FIG. 4A)

When switching element 72 is on, an oscillating current (resonancecurrent) I is supplied from rectifier 40. In this condition, there isestablished a relation among voltage V51 of smoothing capacitor 51,voltage V76 of distortion improving capacitor 76, and voltage Vin of theinput voltage to the inverter (output voltage of rectifier 40) thatVin≧V51+V76.

Mode 2 (FIG. 4B)

After switching element 72 goes off, the oscillating current I continuesto flow through parasitic diode of switching element 71 to chargesmoothing capacitor 51. Then, switching element 71 is triggered to turnon.

Mode 3 (FIG. 4C)

When leakage inductance 83 of output transformer 80 is exhausted in mode2, the oscillating current I is reversed due to the charge accumulatedin capacitors 76 and 75, while discharging capacitor 76.

Mode 4 (FIG. 4D)

When capacitor 76 is discharged, diode 74 becomes conductive to flow theoscillating current I continuously.

Mode 5 (FIG. 4E)

After switching element 71 is turned off, the oscillating current Icontinues to flow through parasitic diode of switching element 72 tocharge smoothing capacitor 51. Then, switching element 72 is triggeredto turn on.

Mode 6 (FIG. 4F)

When leakage inductance 83 of output transformer is exhausted in mode 5,the oscillating current I is reversed due to the charge accumulated insmoothing capacitor 51, thereby charging capacitor 76. When capacitor 76is charged to such an extent of satisfying the relation thatVin=V51+V76, the operation goes back to mode 1.

Therefore, by setting that V51>Vin (peak), and V76 (peak)=V51, the abovesix modes repeats many times during one complete cycle of the AC voltagefrom the AC mains, thereby suppressing harmonics (input currentdistortion) in the input current. In other words, the input current fromthe AC mains is drawn to flow into the inverter even when the AC voltageis at a lowered level within the cycle of the AC voltage, therebysuppressing the harmonics in the input current. In this sense, theinverter having the circuit configuration of FIG. 3 can be referred toas the inverter of a charge-pump type inherently capable of suppressingthe harmonics without requiring additional harmonic suppressing circuitsuch as a passive filter and an active filter. Thus, the use of theinverter of the charge-pump type can reduce the number of the parts,making the whole ballast assembly into a compact design.

The control circuit 100 is designed to control the switching elements 71and 72 in order to preheat, start, and operate the discharge lamps, andalso to deactivate the inverter upon detection of an abnormal conditionor no-load condition. As shown in FIG. 5, the control circuit 100comprises a VCO (voltage-controlled oscillator) 101 which provides todriver IC 110 a series of oscillating pulses of which frequency isvaried with an applied voltage. The driver IC 110 is responsive to theoscillating pulses from VCO 101 for generating driving voltages forturning on the switching elements 71 and 72, details of which will bediscussed later. The control circuit 100 includes a first timer 102, asecond timer 103, an abnormal condition determining circuit 104, and ano-load condition determining circuit 105. Further, the control circuit100 includes a dropping resistor (not shown) which is connected to therectifier 40 for deriving a DC voltage for energizing the controlcircuit at the time of starting the inverter. Thereafter, the controlcircuit 100 is continuously energized by the voltage from the firstauxiliary winding 84. The parts forming the circuit of the controller100 are integrated together with the driver IC 110 into a single chip100A mounted on the bottom surface of the circuit board 10. The firsttimer 102 is programmed to provide to VCO 101 voltages respectivelycontrolling driver IC 110 to operate the inverter for preheating,starting, and operating the discharge lamps in accordance with apredetermined time schedule. The abnormal condition determining circuit104 is connected to monitor the output voltage of the inverter so thatit determines an abnormal condition and issue an abnormal signal whenthe monitored output voltage becomes critically high due to theincreased lamp voltage seen at the end of its operation life. Uponreceiving the abnormal signal, the first timer 102 operates to cease theinverter for a predetermined time period after which it actuates torestart the inverter. The abnormal signal is also fed to the secondtimer 103 which measures a time during which the abnormal signal lastsand issues a stop signal when the measured time exceeds over apredetermined level. The stop signal is fed to deactivate VCO 101 so asto stop driving the switching elements. The no-load conditiondetermining circuit 105 is coupled in the circuit to monitor whether thedischarge lamps are mounted correctly, and issue a no-load signal whenany of the two discharge lamps is dismounted. Upon receiving the no-loadsignal, the first timer 102 operates to deactivate the VCO 101 forceasing the inverter.

The output voltage of the inverter is monitored by a voltage monitorwhich comprises, in addition to the second auxiliary winding 85, adetection circuit 90 having a resistor network of resistors 91 and 92, adiode 93, and a capacitor 94. The resistor network is cooperative withcapacitor 94 to provide a divided DC voltage indicative of the outputvoltage of the inverter to the abnormal condition determining circuit104. The third auxiliary winding 86 is connected to another detectioncircuit 99 which is a resistor network composed of resistors 95 to 98connected in series with the filaments of discharge lamps LA1 and LA2 ina path leading from one end of the rectifier 40 to the no-load conditiondetermining circuit 105 of the control circuit 100. When either or bothof the discharge lamps LA1 and LA2 is disconnected, no current will flowthrough the resistor network of resistors 95 to 99. Upon thisoccurrence, the control circuit 100 responds to stop the inverter.

Now referring to FIG. 6, the driver IC 110 includes a level-shiftcircuit 111, and a pair of buffer circuits 112 and 113. The level-shiftcircuit 111 is provided for generating a high driving voltage to turn onthe high-side switching element 71, and comprise a pulse generator 114,a pair of transistors 115 and 116, and a flip-flop 117. The signal fromthe control circuit 100 is processed at the level-shift circuit 111 andthrough the buffer circuit 112 to provide the high driving voltage at aterminal HO for turning on the high-side switching element 71 of theinverter. While the same signal from the control circuit 100 isprocessed through the buffer circuit 113 to provide a low drivingvoltage at terminal LO for turning on the low-side switching element 72of the inverter. FIG. 7 illustrates waveforms at points A to F in thecircuit of FIG. 6.

Turning back to FIGS. 1 and 2, there are shown which parts of theballast circuit are mounted on which portions of the circuit board 10.The circuit board 10 is of an elongated configuration having a uniformwidth and is formed on its bottom surface with a circuit patternconnecting the parts of the ballast circuit. It is noted that bulk partsare mounted on the top surface of the circuit board, while chipcomponents are mounted on the bottom surface of the circuit board. Asseen in FIG. 1, the bulk parts include input terminal socket 20,filtering capacitor 41, filter 42, capacitors 61, 62 and 76, switchingtransistors 71 and 72, a combination of diodes 73 and 74, inductor 52,smoothing capacitor 51, resistor 65, variable resistors 67 and 68, DCblocking capacitor 75, output transformer 80, capacitors 77, 87 to 89,and the output terminal socket 30. As seen in FIG. 2, the chipcomponents include chip resistors 91, 92, 95 to 98, chip capacitor 94,chip diodes 93 and 66, chip transistor (FET) 64, chip package 100Aintegrating a portion of the control circuit 100 and the driver IC 110,and chip components 100B forming the remainder of the control circuit100.

The input terminal socket 20 and the output terminal socket 30 aremounted at the distal longitudinal ends of the circuit board 10. Each ofthe terminal sockets 20 and 30 has a rapid wire connection and releasemechanism for easy wiring connection and disconnection. The outputtransformer 80 is mounted on one side of the output terminal socket 30in such a manner as to occupy full width of the circuit board 10. Thatis, output transformer 80 has a maximum width among the other bulk partsand the circuit board 10 is dimensioned to have the width approximatelyequal to the width of the output transformer 80, as shown in FIG. 8.Capacitors 77, 87 to 89, each in the form of a film capacitor, arecollectively mounted between the output terminal socket 30 and theoutput transformer 80 such that the film capacitors which develop onlyrelatively low amount of heat can be closely packed into a small spacewithout giving substantial thermal effect to each other.

As discussed hereinbefore with reference to FIG. 3, the detectioncircuit 90 for monitoring the high frequency output voltage of theinverter and providing the detected voltage to the control circuit 100are realized by diode 93, resistors 91 and 92, and capacitor 94. Allthese parts are presented as the chip components and are mounted on thebottom surface of the circuit board 10 at a location immediatelyopposite of output transformer 80, as shown in FIG. 2, in order to bestutilize a space behind the output transformer for adding the detectioncircuit to the ballast. Further, resistors 95 to 98 forming thedetection circuit 99 for detecting whether the discharge lamps aremounted in the portion opposite of output transformer 80 by bestutilization of the space behind the output transformer. In FIG. 2,soldered lead ends of input terminal socket 20, output terminal socket30, output transformer 80, switching elements 71 and 72, and combination73A of diodes 73 and 74 are indicated respectively by numerals 20X, 30X,71X, 72X, and 73X.

Turning back to FIG. 1, smoothing capacitor 51 is mounted at thelongitudinal center of the circuit board 10 so as to be less thermallyaffected from output transformer 80 as well as from a set of capacitors61, 62, and 76 which are mounted at one longitudinal end adjacent to theinput terminal socket 20. Since the smoothing capacitor 51 is in theform of an aluminum electrolytic capacitor of which life is shortenedwhen exposed to high temperature, the above arrangement is advantageousto elongate the life of the smoothing capacitor. Switching elements 71and 72 are each molded into a flat resin package and are mounted at aportion intermediate the longitudinal ends of circuit board 10 andarranged side-by-side along one lateral edge of the circuit board. Also,the combination of diodes 73 and 74 is molded into a like flat resinpackage 73A which is mounted in an adjacent relation to the switchingelement 72. In this connection, the chip package 10A, the chipcomponents 100B of control circuit 100, and associated chip resistors 77and 78 are collectively mounted on the bottom and at near thelongitudinal center of the circuit board in a spaced relation from theswitching transistors 71 and 72, the control circuit can be wellprotected from being influenced by the switching elements and thereforebe free from noises, in addition to that the control circuit and itsassociated resistors can be densely packed on the bottom of the circuitboard 10.

The flat package 71, 72, and 73A are secured to one side wall 212 of thelower case 210 by means of spring clips 250, as shown in FIGS. 9 to 11.As shown in FIG. 1, the side wall 212 is formed with a recess 213 ofwhich bottom projects inwardly of the lower case. It is this recess 213that serves to hold the packages 71, 72, and 73A and to release the heatthereof into the lower case 210. That is, the major surface of eachpackage is held in an intimate contact with the bottom of the recess 213to release the heat to the lower case. The spring clip 250 comprises apair of resilient legs 251 and 252 connected by a thin flat member 253.As shown in FIG. 11, the spring clip 250 serves to retain the upper endof the package 71 (72, 73A) together with the bottom of recess 213between resilient legs 251 and 252, and with the member 253 in contactwith the upper end of the package. When the lower case 210 is coveredwith upper case 220, member 253 comes into contact with the top 222 ofupper case 220, thereby serving to escape the heat of the package alsointo the upper case 220 and therefore improving heat radiation of thepackage. As shown in FIG. 11, the thickness of the resilient leg 251 isless than the depth of recess 213 so that the clip does not add an extrawidth to the whole assembly. The side wall 222 of the upper case 220 isdimensioned to cover the recess 213. It is noted in this connection thatthe height of the package as measured from circuit board 10 plus thethickness of member 253 of spring clip 250 is nearly equal to the heightof output transformer 80. Thus, the height of the assembly is determinedby a higher one of the output transformer and the package.

The variable resistors 67 and 68 are included in the control circuit 100in order to adjust the oscillating frequency of the inverter inaccordance with differing source AC voltage available at different areasof the world. Particularly, variable resistor 67 is provided to adjustthe oscillating frequency at the time of starting the discharge lamps,while variable resistor 68 adjusts the oscillating frequency at thestable-state operation of the discharge lamps. With the inclusion ofvariable resistors, it is possible to provide constant luminous outputof the discharge lamps irrespective of the different source AC voltages,while minimizing an electrical stress applied to the parts of theinverter.

As shown in FIG. 1, the dielectric sheet 230 is designed to cover almostall of the circuit board 10 and comprises an elongated bottom 231, apair of side walls 232, and a top extension 233 extending one of theside walls. The top extension 233 is bent over the parts mounted on thecircuit board 10 so as to cover at least the entire top of outputtransformer 80, capacitors 77, 87 to 89, smoothing capacitor 51,capacitor 75, resistor 65, and variable resistors 67 and 68. Capacitor41 as well as filter 42 which have sufficient insulation capabilityagainst the upper case 220 can be left uncovered by the extension 233 soas to improve heat radiation from these parts to the upper case 230. Oneof the side walls 232 has a cut-out 235 at a portion corresponding tothe recess 213 of the lower case 210 for contacting the packages 71, 72,and 73A directly with the bottom of recess 213. Projecting integrally onthe bottom 231 of dielectric sheet 230 are longitudinally spaced studs236 to support the circuit board 10, improving electrical insulation ofthe circuit board 10 from the bottom of the lower case 210. The studs236 are formed to abut against the bottom of circuit board 10 atportions free from the lead ends of the parts mounted on top of thecircuit board. Thus, the dielectric sheet 230 can be protected frombeing scratched by the lead ends of the parts on top of circuit board10. One of the studs 236 is located to support the output transformer 80which is a heaviest part of the ballast. It is noted in this connectionthat the lead ends of the parts project on the bottom surface of thecircuit board 10 so as not to exceed a maximum height of the chips 64and 100A mounted on the bottom of circuit board 10.

As shown in FIG. 12, the circuit board 10 is formed on its bottom with apair of parallel lines P1 and P2 forming a main current path carryingthe high frequency output current to the output transformer 80, and witha ground line P3 of the inverter. The chip components of the controlcircuit 100 are mounted in an area X which is isolated from the lines P1and P2 by the ground line P3. Thus, the control circuit can be wellprotected from being adversely affected by the high frequency current.Further, the main current path can be closely arranged to form a closedcircuit occupying an area which is small enough to reduce radiationnoise from the main current path.

As shown in FIG. 13, all the chips including the chip package 100A ofthe control circuit 100 mounted on the bottom of the circuit board 10are oriented uniformly such that terminals or electrodes D at theopposite ends of each part are oriented in a width direction of thecircuit board 10. This is advantageous for reliable soldering connectionof the electrodes D to the circuit pattern while moving the circuitboard within a soldering bath along the lengthwise direction (indicatedby an arrow) of the circuit board.

As shown in FIG. 1, the circuit board 10 is held on a plurality ofstands 214 which are formed at four corners of the bottom 211 of thelower case 210 as being upturned therefrom. One of the stands 214 has anupright fin 215 which fits into a single slit 11 formed at onelongitudinal and width end of the circuit board 10. As best shown inFIG. 14, the upright fin 215 of the one stand 214 is made longer thanthose of the other three stands 214 such that the circuit board 10 canbe correctly positioned on the bottom of the lower case 210, while theupright fins of the other three stands abut against the longitudinalends of the circuit board, as shown in FIG. 15. Thus, the stand 214 withthe long fin 215 is cooperative with the slit 11 to define a positioningmeans for correct assembly of the circuit board in the casing.Alternately, as shown in FIG. 16, the slits 11 may be formedrespectively at the longitudinal ends of the circuit board 10 but offsettowards one width end thereof so as to receive the correspondingly longupright fins 215 at the longitudinal end of the lower case 210. Further,as shown in FIGS. 17 and 18, one of the stands 214 may be formed with aseparate pin 216 for insertion into a corresponding hole 12 of thecircuit board 10 for correct positioning of the circuit board.

As shown in FIGS. 19 and 20, a plurality of the circuit boards 10 areprepared in the form of an array in which the boards 10 are arrangedside-by-side with a separation line or V-shaped cut S formed between theadjacent ones of the boards. Further, each board 10 is formed to havereduced-in-width section 13 along its length to thereby leavecorresponding slits 14 between the adjacent two boards 10. With theprovision of the slits 14, the reduced-in-width sections can be lesssusceptible to a stress which develops when separating the board 10 fromthe array by bending it along the separation line S. It is thisreduced-in-width section 12 that is reserved for mounting the chipcomponents M which is delicate or fragile so as to protect the chipcomponents from the stress applied at the time of separating the board.The chip components typically include chip-type ceramic capacitors.

Turning back to FIG. 1, the upper case 220 has a pair of upper sidewalls 222 depending from the lateral ends the top 221 to overlap thelower side wall 212 of the lower case 210. The upper side walls 222 areprovided respectively with inward projections 223 which engage intocorresponding openings 217 in the lower side walls 212 of the lower case210 to secure the upper case to the lower case. Depending respectivelyat the longitudinal ends of the upper case 220 are partitions 224 and225 which are positioned behind the input and output terminal sockets 20and 30 to conceal the other parts of the ballast in the tubular casing.

1. An electronic ballast for a discharge lamp comprising: an AC-DCconverter adapted to be connected to receive a source AC voltage andconvert the source AC voltage into a DC voltage; an inverter providing ahigh frequency output voltage from said DC voltage for operating thedischarge lamp, said inverter comprising at least one switching element,a control circuit for said switching element, and an output transformeradapted to be connected to said discharge lamp, said control circuitactivating said switching element to repetitively interrupt said DCvoltage to provide the high frequency output voltage to said dischargelamp through said output transformer; a single circuit board mountingthereon electric parts which constitute said converter and saidinverter; a tubular casing accommodating therein said circuit board; anda dielectric sheet interposed between said circuit board and said casingfor insulation of said circuit board and said electric parts from saidcasing, wherein said ballast includes a plurality of chip componentsforming a detection circuit for detection of a condition of saiddischarge lamp, said circuit board mounts said output transformer on itstop surface and mounting said chip components on its bottom surface at alocation immediately opposite of said output transformer, and saiddetection circuit is provided to detect an operating condition of saiddischarge lamp and includes a resistor network composed of chipresistors for dividing the output voltage of said inverter into adetection voltage which is fed to said control circuit for control ofsaid inverter.
 2. The ballast as set forth in claim 1, wherein saiddetection circuit further includes a chip capacitor which issurface-mounted on the bottom surface of said circuit board togetherwith said chip resistors.
 3. An electronic ballast for a discharge lampcomprising: an AC-DC converter adapted to be connected to receive asource AC voltage and convert the source AC voltage into a DC voltage;an inverter providing a high frequency output voltage from said DCvoltage for operating the discharge lamp, said inverter comprising atleast one switching element, a control circuit for said switchingelement, and an output transformer adapted to be connected to saiddischarge lamp, said control circuit activating said switching elementto repetitively interrupt said DC voltage to provide the high frequencyoutput voltage to said discharge lamp through said output transformer; asingle circuit board mounting thereon electric parts which constitutesaid converter and said inverter; a tubular casing accommodating thereinsaid circuit board; and a dielectric sheet interposed between saidcircuit board and said casing for insulation of said circuit board andsaid electric parts from said casing, wherein said ballast includes aplurality of chip components forming a detection circuit for detectionof a condition of said discharge lamp, said circuit board mounts saidoutput transformer on its top surface and mounting said chip componentson its bottom surface at a location immediately opposite of said outputtransformer, and said detection circuit is provided for detection ofwhether said discharge lamp is connected to said inverter, saiddetection circuit being in the form of a resistor network which iscomposed of chip resistors, and said chip resistors beingsurface-mounted on the bottom of said circuit board immediately oppositeof said output transformer.
 4. An electronic ballast for a dischargelamp comprising: an AC-DC converter adapted to be connected to receive asource AC voltage and convert the source AC voltage into a DC voltage;an inverter providing a high frequency output voltage from said DCvoltage for operating the discharge lamp, said inverter comprising atleast one switching element, a control circuit for said switchingelement, and an output transformer adapted to be connected to saiddischarge lamp, said control circuit activating said switching elementto repetitively interrupt said DC voltage to provide the high frequencyoutput voltage to said discharge lamp through said output transformer; asingle circuit board mounting thereon electric parts which constitutesaid converter and said inverter; a tubular casing accommodating thereinsaid circuit board; and a dielectric sheet interposed between saidcircuit board and said casing for insulation of said circuit board andsaid electric parts from said casing, wherein said ballast includes aplurality of chip components forming a detection circuit for detectionof a condition of said discharge lamp, said circuit board mounts saidoutput transformer on its top surface and mounting said chip componentson its bottom surface at a location immediately opposite of said outputtransformer, and said output transformer is of a flat configuration,said dielectric sheet having an extension which covers the entire topface of said output transformer for insulation thereof from said casing.5. An electronic ballast for a discharge lamp comprising: an AC-DCconverter adapted to be connected to receive a source AC voltage andconvert the source AC voltage into a DC voltage; an inverter providing ahigh frequency output voltage from said DC voltage for operating thedischarge lamp, said inverter comprising at least one switching element,a control circuit for said switching element, and an output transformeradapted to be connected to said discharge lamp, said control circuitactivating said switching element to repetitively interrupt said DCvoltage to provide the high frequency output voltage to said dischargelamp through said output transformer; a single circuit board mountingthereon electric parts which constitute said converter and saidinverter; a tubular casing accommodating therein said circuit board; anda dielectric sheet interposed between said circuit board and said casingfor insulation of said circuit board and said electric parts from saidcasing, wherein said ballast includes a plurality of chip componentsforming a detection circuit for detection of a condition of saiddischarge lamp, said circuit board mounts said output transformer on itstop surface and mounting said chip components on its bottom surface at alocation immediately opposite of said output transformer, and saidinverter includes a smoothing capacitor which accumulates therein saidDC voltage from said converter, and further includes a set of capacitorswhich generate a relatively large amount of heat, said circuit boardbeing of an elongated configuration and mounting said output transformerand a set of said capacitors on its top surface at its oppositelongitudinal ends thereof, respectively, said smoothing capacitor beingmounted on the top surface of said circuit board intermediate betweenthe longitudinal ends thereof.
 6. The ballast as set forth in claim 5,wherein said casing is of an elongated configuration and is composed ofa lower case having a base and a pair of lower side walls upstandingfrom the opposite lateral sides of the base, and an upper case having atop and a pair of upper side walls depending from the opposite lateralsides of the top to overlap the lower side walls, respectively, saidswitching element being molded into a flat resin package and beingmounted on said circuit board intermediate the opposite longitudinalends and at one lateral side of said circuit board to project on the topsurface of said circuit board, said resin package having a major surfacebeing held in intimate contact with one of said lower side walls forescaping heat generated at the switching element to the lower case. 7.The ballast as set forth in claim 6, wherein the one of said lower sidewalls is formed with a recess of which bottom projects inwardly of saidlower case, said recess being located intermediate the longitudinal endsof the lower case, said package being secured to the bottom of saidrecess by means of a spring clip for making said major surface of saidpackage intimate contact with the bottom of said recess, said springclip being shaped into a U-shaped configuration having a pair ofresilient legs connected by a thin flat member for pinching an upperportion of the package and the bottom of said recess between said legs,said thin flat member being held in intimated contact with a top of saidpackage as well as with said top of said upper case for escaping theheat generated at the package also to said upper case through saidmember, said resilient leg having a thickness which is smaller than adepth of said recess.
 8. An electronic ballast for a discharge lampcomprising: an AC-DC converter adapted to be connected to receive asource AC voltage and convert the source AC voltage into a DC voltage;an inverter providing a high frequency output voltage from said DCvoltage for operating the discharge lamp, said inverter comprising atleast one switching element, a control circuit for said switchingelement, and an output transformer adapted to be connected to saiddischarge lamp, said control circuit activating said switching elementto repetitively interrupt said DC voltage to provide the high frequencyoutput voltage to said discharge lamp through said output transformer; asingle circuit board mounting thereon electric parts which constitutesaid converter and said inverter; a tubular casing accommodating thereinsaid circuit board; and a dielectric sheet interposed between saidcircuit board and said casing for insulation of said circuit board andsaid electric parts from said casing, wherein said ballast includes aplurality of chip components forming a detection circuit for detectionof a condition of said discharge lamp, said circuit board mounts saidoutput transformer on its top surface and mounting said chip componentson its bottom surface at a location immediately opposite of said outputtransformer, and said inverter includes a pair of switching elementsconnected in series across an output of said AC-DC converter, saidcontrol circuit including a driver having a level-shift circuit forproviding a high driving voltage to turn on a high side one of saidswitching elements, said control circuit being integrated together withsaid driver into a single chip, said chip being mounted on the bottomsurface of said circuit board at a location intermediate between thelongitudinal ends of said circuit board.
 9. The ballast as set forth inclaim 5, wherein said inverter includes a plurality of capacitors eachin the form of a film capacitor, said circuit board being provided onits top surface at the longitudinal ends respectively with an inputterminal socket for connection of said converter to said source ACvoltage and an output terminal socket for connection of the outputtransformer to said discharge lamp, said output transformer beingdisposed adjacent to said output terminal, said film capacitors beingmounted collectively on said circuit board between said outputtransformer and said output terminal socket.
 10. The ballast as setforth in claim 4, wherein said circuit board is of an elongatedconfiguration having an approximately uniform width which is nearlyequal to a width of said output transformer, the width of saidtransformer being largest among all of said electric parts.
 11. Theballast as set forth in claim 8, wherein said circuit board is formed onits bottom surface with a ground line for said inverter, and with a maincurrent path carrying a high frequency current to said outputtransformer, said chip of said control circuit being isolated from saidmain current path by said ground line, and said main current pathincluding a pair of main current lines major portions of which run inclose parallel relation to each other.
 12. An electronic ballast for adischarge lamp comprising: an AC-DC converter adapted to be connected toreceive a source AC voltage and convert the source AC voltage into a DCvoltage; an inverter providing a high frequency output voltage from saidDC voltage for operating the discharge lamp, said inverter comprising atleast one switching element, a control circuit for said switchingelement, and an output transformer adapted to be connected to saiddischarge lamp, said control circuit activating said switching elementto repetitively interrupt said DC voltage to provide the high frequencyoutput voltage to said discharge lamp through said output transformer; asingle circuit board mounting thereon electric parts which constitutesaid converter and said inverter; a tubular casing accommodating thereinsaid circuit board; and a dielectric sheet interposed between saidcircuit board and said casing for insulation of said circuit board andsaid electric parts from said casing, wherein said ballast includes aplurality of chip components forming a detection circuit for detectionof a condition of said discharge lamp, said circuit board mounts saidoutput transformer on its top surface and mounting said chip componentson its bottom surface at a location immediately opposite of said outputtransformer, and said circuit board is of an elongated configuration andis separated from an array in which a plurality of said circuit boardsare arranged side-by-side with a separation line between the adjacentones of said circuit boards, each of said circuit boards having areduced-in-width section within the length thereof to define a slitalong said separation line between the adjacent circuit boards, saidinverter including at least one capacitor in the form of a chip-typeceramic capacitor, said chip-type ceramic capacitor being mounted onsaid reduced-in-width section.
 13. An electronic ballast for a dischargelamp comprising: an AC-DC converter adapted to be connected to receive asource AC voltage and convert the source AC voltage into a DC voltage;an inverter providing a high frequency output voltage from said DCvoltage for operating the discharge lamp, said inverter comprising atleast one switching element, a control circuit for said switchingelement, and an output transformer adapted to be connected to saiddischarge lamp, said control circuit activating said switching elementto repetitively interrupt said DC voltage to provide the high frequencyoutput voltage to said discharge lamp through said output transformer; asingle circuit board mounting thereon electric parts which constitutesaid converter and said inverter; a tubular casing accommodating thereinsaid circuit board; and a dielectric sheet interposed between saidcircuit board and said casing for insulation of said circuit board andsaid electric parts from said casing, wherein said ballast includes aplurality of chip components forming a detection circuit for detectionof a condition of said discharge lamp, said circuit board mounts saidoutput transformer on its top surface and mounting said chip componentson its bottom surface at a location immediately opposite of said outputtransformer, and said circuit board is of an elongated configuration,said electric parts including a plurality of chips each having terminalson opposite ends thereof, respectively, said chips being mounted on thebottom surface of said circuit board with said terminals oriented in awidth direction of said circuit board.
 14. An electronic ballast for adischarge lamp comprising: an AC-DC converter adapted to be connected toreceive a source AC voltage and convert the source AC voltage into a DCvoltage; an inverter providing a high frequency output voltage from saidDC voltage for operating the discharge lamp, said inverter comprising atleast one switching element, a control circuit for said switchingelement, and an output transformer adapted to be connected to saiddischarge lamp, said control circuit activating said switching elementto repetitively interrupt said DC voltage to provide the high frequencyoutput voltage to said discharge lamp through said output transformer; asingle circuit board mounting thereon electric parts which constitutesaid converter and said inverter; a tubular casing accommodating thereinsaid circuit board; and a dielectric sheet interposed between saidcircuit board and said casing for insulation of said circuit board andsaid electric parts from said casing, wherein said ballast includes aplurality of chip components forming a detection circuit for detectionof a condition of said discharge lamp, said circuit board mounts saidoutput transformer on its top surface and mounting said chip componentson its bottom surface at a location immediately opposite of said outputtransformer, and said dielectric sheet is provided with a plurality ofstuds for supporting said circuit board.
 15. An electronic ballast for adischarge lamp comprising: an AC-DC converter adapted to be connected toreceive a source AC voltage and convert the source AC voltage into a DCvoltage; an inverter providing a high frequency output voltage from saidDC voltage for operating the discharge lamp, said inverter comprising atleast one switching element, a control circuit for said switchingelement, and an output transformer adapted to be connected to saiddischarge lamp, said control circuit activating said switching elementto repetitively interrupt said DC voltage to provide the high frequencyoutput voltage to said discharge lamp through said output transformer; asingle circuit board mounting thereon electric parts which constitutesaid converter and said inverter; a tubular casing accommodating thereinsaid circuit board; and a dielectric sheet interposed between saidcircuit board and said casing for insulation of said circuit board andsaid electric parts from said casing, wherein said ballast includes aplurality of chip components forming a detection circuit for detectionof a condition of said discharge lamp, said circuit board mounts saidoutput transformer on its top surface and mounting said chip componentson its bottom surface at a location immediately opposite of said outputtransformer, and said case is provided with positioning means forholding said circuit board in a correct position.
 16. An electronicballast for a discharge lamp comprising: an AC-DC converter adapted tobe connected to receive a source AC voltage and convert the source ACvoltage into a DC voltage; an inverter providing a high frequency outputvoltage from said DC voltage for operating the discharge lamp, saidinverter comprising at least one switching element, a control circuitfor said switching element, and an output transformer adapted to beconnected to said discharge lamp, said control circuit activating saidswitching element to repetitively interrupt said DC voltage to providethe high frequency output voltage to said discharge lamp through saidoutput transformer; a single circuit board mounting thereon electricparts which constitute said converter and said inverter; a tubularcasing accommodating therein said circuit board; and a dielectric sheetinterposed between said circuit board and said casing for insulation ofsaid circuit board and said electric parts from said casing, whereinsaid ballast includes a plurality of chip components forming a detectioncircuit for detection of a condition of said discharge lamp, saidcircuit board mounts said output transformer on its top surface andmounting said chip components on its bottom surface at a locationimmediately opposite of said output transformer, and said outputtransformer includes a first auxiliary winding which provides a drivingvoltage for energizing said control circuit, a second auxiliary windingwhich provides a detection output indicative of the high frequencyoutput voltage for monitoring the operation of said inverter, and athird auxiliary winding which provides a preheat current to filaments ofsaid discharge lamp.
 17. The ballast as set forth in claim 16, whereinsaid output transformer is a leakage transformer having a primarywinding connected in circuit to flow a high frequency current, and asecondary winding magnetically coupled to said primary winding to inducea resulting high frequency voltage which is applied to said dischargelamp, said first auxiliary winding being coupled to said primarywinding, said second and third auxiliary windings being coupled to saidsecondary winding.
 18. An electronic ballast for a discharge lampcomprising: an AC-DC converter adapted to be connected to receive asource AC voltage and convert the source AC voltage into a DC voltage;an inverter providing a high frequency output voltage from said DCvoltage for operating the discharge lamp, said inverter comprising atleast one switching element, a control circuit for said switchingelement, and an output transformer adapted to be connected to saiddischarge lamp, said control circuit activating said switching elementto repetitively interrupt said DC voltage to provide the high frequencyoutput voltage to said discharge lamp through said output transformer; asingle circuit board mounting thereon electric parts which constitutesaid converter and said inverter; a tubular casing accommodating thereinsaid circuit board; and a dielectric sheet interposed between saidcircuit board and said casing for insulation of said circuit board andsaid electric parts from said casing, wherein said ballast includes aplurality of chip components forming a detection circuit for detectionof a condition of said discharge lamp, said circuit board mounts saidoutput transformer on its top surface and mounting said chip componentson its bottom surface at a location immediately opposite of said outputtransformer, and said AC-DC converter includes a rectifier providing therectified DC voltage, said control circuit including a dropping resistorwhich is connected to said rectifier for deriving a DC voltage forenergizing said control circuit at the time of starting the inverter.19. An electronic ballast for a discharge lamp comprising: an AC-DCconverter adapted to be connected to receive a source AC voltage andconvert the source AC voltage into a DC voltage; an inverter providing ahigh frequency output voltage from said DC voltage for operating thedischarge lamp, said inverter comprising at least one switching element,a control circuit for said switching element, and an output transformeradapted to be connected to said discharge lamp, said control circuitactivating said switching element to repetitively interrupt said DCvoltage to provide the high frequency output voltage to said dischargelamp through said output transformer; a single circuit board mountingthereon electric parts which constitute said converter and saidinverter; a tubular casing accommodating therein said circuit board; anda dielectric sheet interposed between said circuit board and said casingfor insulation of said circuit board and said electric parts from saidcasing, wherein said ballast includes a plurality of chip componentsforming a detection circuit for detection of a condition of saiddischarge lamp, said circuit board mounts said output transformer on itstop surface and mounting said chip components on its bottom surface at alocation immediately opposite of said output transformer, and saidinverter is of a charge-pump type having a capacitor which is connectedin a path of receiving an input current from said AC-DC converter andflowing the input current through an oscillating element of the inverterand through the switching element for suppressing harmonics in the inputcurrent from said source AC voltage.
 20. The ballast as set forth inclaim 19, wherein said ballast includes an adjusting means mounted onsaid circuit board to adjust output characteristics of said dischargelamp.
 21. The ballast as set forth in claim 20, wherein said adjustormeans operates to adjust a lamp start output power and a lamp operatingoutput power fed to said discharge lamp respectively at the time ofstarting the lamp and during the steady-state operation of the lamp. 22.An electronic ballast for a discharge lamp comprising: an AC-DCconverter adapted to be connected to receive a source AC voltage andconvert the source AC voltage into a DC voltage; an inverter providing ahigh frequency output voltage from said DC voltage for operating thedischarge lamp, said inverter comprising at least one switching element,a control circuit for said switching element, and an output transformeradapted to be connected to said discharge lamp, said control circuitactivating said switching element to repetitively interrupt said DCvoltage to provide the high frequency output voltage to said dischargelamp through said output transformer; a single circuit board mountingthereon electric parts which constitute said converter and saidinverter; a tubular casing accommodating therein said circuit board; anda dielectric sheet interposed between said circuit board and said casingfor insulation of said circuit board and said electric parts from saidcasing, wherein said ballast includes a plurality of chip componentsforming a detection circuit for detection of a condition of saiddischarge lamp, said circuit board mounts said output transformer on itstop surface and mounting said chip components on its bottom surface at alocation immediately opposite of said output transformer, and saiddetection circuit includes a resistor network composed of chip resistorsfor dividing the output voltage of said inverter into a detectionvoltage which is fed to said control circuit for control of saidinverter, said detection circuit also including a chip capacitor whichis surface-mounted on the bottom surface of said circuit board togetherwith said chip resistors, said ballast further including anotherdetection circuit in the form of a resistor network which is composed ofchip resistors for detection of whether said discharge lamp is connectedto said inverter, said chip resistors being surface-mounted on thebottom of said circuit board immediately opposite of said outputtransformer, said inverter including a pair of switching elementsconnected in series across an output of said AC-DC converter, saidcontrol circuit including a driver having a level-shift circuit forproviding a high driving voltage to turn on a high side one of saidswitching elements, said control circuit being integrated together withsaid driver into a single chip, said chip being mounted on the bottomsurface of said circuit board at a location intermediate thelongitudinal ends of said circuit board, and said inverter being of acharge-pump type having a capacitor which is connected in a path ofreceiving an input current from said AC-DC converter and flowing theinput current through an oscillating element of the inverter and throughthe switching element for suppressing harmonics in the input currentfrom said source AC voltage.